1. Field of the Invention
This invention relates to a video signal processing apparatus, and more particularly, to an apparatus adapted for processing a video signal including line-sequential signals.
2. Description of the Prior Art
Generally, in rearranging line-sequential signals into concurrent or simultaneous signals, the kinds of the line-sequential signals must be discriminated from each other for every horizontal scanning period (hereinafter referred to as an H period). Therefore, in recording or transmitting these signals, they are arranged into some signal forms that permit discrimination of their kinds. For example, in line-sequentially recording signals of two different kinds, they have been arranged to have their DC components offset at every two H periods, to be frequency offset or to have a flag signal added at every interval of two H periods.
However, when the kinds of the line-sequential signals which have been processed in this manner are to be discriminated, one from the other, by a reproduction system, the adverse effects of drop-outs, transmission distortion, etc. prevent accurate discrimination. The details of this problem will be understood from the following description of an example of the apparatus of the kind arranged to reproduce a still picture by continuously reproducing a video signal which has one field portion thereof recorded in each circular recording track formed on a magnetic sheet and includes line-sequential color difference signals with a DC offset arranged at every interval of two H periods:
FIG. 1 of the accompanying drawings shows in a block diagram the essential arrangement of the conventional reproducing apparatus of the above-stated kind. FIGS. 2(a) to 2(g) show the waveforms at various points (a) to (g) shown in FIG. 1. Referring to FIG. 1, line sequential color difference signals obtained from a reproduced video signal are arranged to be supplied to a terminal t1. A horizontal synchronizing signal obtained from the reproduced video signal is arranged to be supplied to another terminal t2. The reproducing apparatus comprises a sample-and-hold circuit 1; an amplifier 2 arranged to amplify the output of the sample-and-hold circuit 1; a comparator 3 arranged to compare the output of the amplifier 2 with a predetermined level; a D type flip-flop 4 (hereinafter referred to as DFF) arranged to receive the output of the comparator 3 as a data input and is triggered by the fall of the horizontal synchronizing signal, which is as shown in FIG. 2(d); a delay line 6 for delaying by 1 H period, which is 63.556 .mu.sec; a monostable multivibrator 7 arranged to be triggered by the horizontal synchronizing signal (d) and to form a signal as shown in FIG. 2(b); and switches SW1 and SW2. The contact piece E of the switch SW1 is connected to one contact A thereof and the contact piece F of the switch SW2 to one contact C thereof when the signal produced from the DFF 4 is at a high level. The contact piece E is connected to the other contact B of the switch SW1 while the contact piece F is connected to the other contact D when the signal is at a low level. The color difference signals are thus rearranged into line simultaneous signals. The line simultaneous color difference signals are respectively supplied to terminals t3 and t4.
Let us assume that the line-sequential color difference signals include a signal of red (R)-luminance (Y) and a signal of blue (B)-luminance (Y) and that the color difference signal R-Y has a higher center level than the other color difference signal B-Y, and is recorded in a line offset manner. In this instance, if the signal R-Y is reproduced during a certain H period, a signal which is sampled and held by the fall of the signal (b) of the monostable multivibrator 7 is at a high level during this period. Accordingly, a signal (e) produced from the DFF 4 during a next H period is at a high level. In other words, if the output signal of the 1 H delay line 6 is the signal R-Y, the switch SW1 has its contact piece E connected to the contact A and the simultaneous signal R-Y is produced from the terminal t3. Meanwhile, a simultaneous signal B-Y is produced likewise from the terminal t4.
However, in obtaining line-simultaneous color difference signals from the line sequential color difference signals by the above-stated arrangement, it becomes rearly impossible to obtain the result of a sample-and-hold operation with correct DC offset in the event of some defect in the line sequential color difference signals obtained at the time of sampling, such as deterioration in the S/N ratio due to a drop-out or the like. In that event, the switching operation of the switches SW1 and SW2 becomes erroneous and the switch-over between the signals R-Y and B-Y might become converse. For example, the deterioration of the S/N ratio resulting from a drop-out A indicated in FIG. 2(a) would result in a converse switch-over operation during a period as indicated by B in FIG. 2(f). This error makes a reproduced picture very disagreeable. For example, a red line would appear on the picture of a blue color only. In the case of a still picture reproducing apparatus in particular, such S/N ratio deterioration due to a drop-out quite likely arises during the same H period. In that instance, a conspicuous line appears always in the same part of the reproduced picture.